this one's easy ... it has nothing to do with how you are wearing it, whether it's too tight, not tight enough, or whether the cross strap is adjusted right. The bag just simply wasn't designed by someone who rides a bike ... looks like it was designed for college students wanting to feel cool by latching on to a fashion statement. sorry to say that, and i'm sure a half a dozen people will say i'm an idiot, but the strap isn't padded, nor does it have any shape to it (so it will stay in place).

buy whatever padded strap cover and hope that works ... if it doesn't, get a messenger bag for people on bikes (i.e. chrome, reload, or PAC ...). Mind you, they are expensive ... but on the bright side, they are really quite comfortable (for much longer than a half-hour as one poster commented) and you probably won't have to buy another one for ten years (or more).